The present invention relates generally to insoles for shoes and particularly to an insole having a plurality of ribs formed along the undersurface thereof.
In most temperate climates, it is not unusual, particularly in the colder seasons, for a significant portion of the population to suffer either continuously or intermittently from cold feet. This occurs not only as a result of cold outside air temperature, but also as a result of increased skin temperature and sweating which follows physical exertion such as walking or running. When sweating occurs, sweat is absorbed by the clothing around the feet. If sufficient air circulation is provided, excess moisture will soon be diverted from the feet. However, if the foot clothing permits insufficient circulation, water vapor will condense at normal room temperatures and the feet will remain damp. Such dampness can be felt for hours, particularly when the feet do not perform any work during sitting and foot skin temperature drops as a result of slight, constant evaporation of excess moisture.
The importance of the micro-climate in the clothed foot to the general health and well-being of the population is the subject of increasing awareness. On the basis of scientific investigation into the bioclimate of the clothed foot conducted by the Work Group at the Institute for Work Physiology of the Institute of Technology Munich between the years 1967 and 1975, it has been determined that human feet stay comfortable when foot skin temperature is maintained between 30.degree. C. and 35.degree. C. at a relative humidity of no more than 65%. Higher relative humidities result in a damp, uncomfortable feeling. Thus, comfort is greatly affected by both the temperature and humidity within the shoe which, in turn, are each subject to a variety of endogenous as well as exogenous influences. Since, in temperate climates, the quantity of sweat produced by each foot can be as great as 70 ml every 12 hours, the physical properties of the foot clothing have a pronounced effect on the micro-climate within the shoe.
Prolonged undercooling of the feet is harmful to health. A long lasting, warm and moist climate at the foot promotes mycosis, eczemas and, on occasion, allergies as well. It is estimated that the majority of adults suffer from dermatosis of the foot as well as various other diseases of the skin. Large indoor swimming pools, saunas, and dressing rooms of swimming and sports facilities provide ample opportunity for such infection to spread. This problem is compounded as various sport and leisure time activities become increasingly popular.
In view of the various foot disorders likely to result from an excessively hot and humid micro-climate within the shoe, it is an important function of foot clothing to help reduce the dampness of the feet. Generally, effective reduction of such dampness requires effective air circulation between the foot and the surrounding foot clothing. However, depending on the particular type of foot clothing, such circulation might only exist to a limited degree.
The present invention is directed to an insole which not only absorbs the sweat of the foot, but which also permits, and promotes, air movement within the shoe to enhance air circulation around the foot. The insole not only permits the movement of air, but also functions to provide a pumping effect which brings about additional air circulation. Since the ability of an insole to directly absorb sweat is limited by the volume of the insole and by the surface area of the cell structure of which it is formed, the additional air circulation provided by the insole of the present invention greatly enhances its ability to carry sweat from the feet and thereby reduce humidity within the shoe.
The insole of the present invention is adapted to maintain a comfortable micro-climate within the shoe both when the foot is at rest and when the foot is moving during walking. Since little heat is generated when the foot is at rest, the insole preferably functions to thermally insulate the feet from cold outside air temperatures. Accordingly, the insole is constructed to minimize the conduction of heat from the sole of the foot to the top-sole of the shoe.
To provide such thermal insulation, the insole can be satisfactorily formed from a fine-pored foam latex. Since still air is a highly effective heat insulator, it is preferable that the foam have cell walls which are as thin as possible. Expressed differently, the specific weight of the foam used in the insole is preferably as low as possible. It will be appreciated however that, in addition to foam latex, other materials, such as closed-pored foams formed of polyvinyl chloride or polyurethane, as well as various felts, non-woven fabrics, cardboard, leather and leather fiber plates, are also heat insulators and can be satisfactorily used.
During, and immediately following physical activity such as walking or running, the conditions within the shoe change markedly from when the foot is at rest. This occurs as a consequence of increased sweating and as a further consequence of increased skin temperature which results from increased blood circulation to the feet. The combination of sweat and increased temperature form water vapor. If not removed from the foot clothing as fast as possible, such water vapor forms a warm, moist micro-environment around the foot which can promote the growth of fungi and microbes. If the outside air temperature is cold, great discomfort may be experienced as a result of the increased dampness.
In view of the foregoing, it is desirable that insoles have sufficient capacity to fully absorb the first discharge of sweat from the feet. The permeability of the insole to water vapor is preferably as great as possible to permit the vapor to be effectively carried from the vicinity of the feet. Additionally, the insole preferably provides for a hollow space or ventilation channel to be formed beneath the sole of the foot in order to permit the free flow of water vapor therethrough such that excess dampness of the foot is avoided.
Permeability to air is a desired attribute of the insole since water vapor is preferably diverted away from the sole of the foot, through the shaft of the shoe, and then to the outside air. While it is preferable for the insole to act as an insulator when the foot is at rest, the increased heat and moisture produced upon physical exertion must not be insulated, but must be diverted by means of thermoconduction, radiation and convection. As heat radiation is generally insignificant, conduction is the primary mechanism by which heat is carried from the shoe. If the stocking and the sole of the shoe are damp, such heat conduction is more effective than when the foot clothing is dry.
Reduced permeability of foot clothing results in relatively greater humidity of the micro-climate within the shoe. Typically, the shoe sole, which consists of an out-sole, an insole, and a top-sole, is the least permeable structure of ordinary foot clothing. Accordingly, the greatest difficulty is encountered in keeping the sole of the foot dry when increased sweating of the foot occurs. Since it is becoming increasingly common to replace some or all of the natural fibers in stockings with synthetic fibers, such stockings are less and less capable of effectively insulating the foot when it is at rest, or of absorbing moisture during, and immediately following, physical activity. Accordingly, as stockings play a smaller and smaller role in maintaining an optimum bioclimatic environment within the shoe, the role played by the insole takes on an increasing importance.